A video showcasing the stool and shower used for cleaning off, an inside pool and an outside pool.

An onsen(温泉) is a Japanese hot spring and the bathing facilities and inns frequently situated around them. As a volcanically active country, Japan has thousands of onsens scattered throughout all of its major islands.

Onsens come in many types and shapes, including outdoor (露天風呂 or 野天風呂,roten-buro or noten-buro) and indoor baths. Baths may be either publicly run by a municipality or privately (内湯,uchiyu), often as part of a hotel, ryokan, or bed and breakfast(民宿,minshuku).

The presence of an onsen is often indicated on signs and maps by the symbol ♨ or the kanji湯 (yu, meaning "hot water"). Sometimes the simpler hiragana character ゆ (yu), understandable to younger children, is used.

Traditionally, onsens were located outdoors, although a large number of inns have now built indoor bathing facilities as well. Onsens by definition use naturally hot water from geothermally heated springs. Onsens are different from sentō, indoor public bath houses where the baths are filled with heated tap water.

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Traditionally, men and women bathed together at both onsens and sentōs, but gender separation has been enforced since the opening of Japan to the West during the Meiji Restoration. Mixed bathing(混浴,kon'yoku) persists at some special onsen in rural areas of Japan,[1] which usually also provide the option of separate "women-only" baths or different hours for the two sexes. Men may cover their genitals with a small towel while out of the water, while women usually wrap their bodies in full-size towels. Children of either sex may be seen in both the men's and the women's baths. In some prefectures of Japan, including Tokyo, where nude mixed bathing is banned, people are required to wear swimsuits or yugi (湯着,yugi), or yuami-gi, which are specifically designed for bathing.

At an onsen, as at a sentō, all guests are expected to wash and rinse themselves thoroughly before entering the hot water. Bathing stations are equipped with stools, faucets, wooden buckets, and toiletries such as soap and shampoo; nearly all onsen also provide removable shower heads for bathing convenience. Entering the onsen while still dirty or with traces of soap on the body is socially unacceptable.[2]

Onsen guests generally bring a small towel with them to use as a wash cloth. The towel can also provide a modicum of modesty when walking between the washing area and the baths. Some onsen allow one to wear the towel into the baths, while others have posted signs prohibiting this, saying that it makes it harder to clean the bath. It is sometimes against the rules to immerse or dip towels in the onsen bath water, since this can be considered unclean. In this latter case, people normally set their towels off to the side of the water when enjoying the baths, or place their folded towels on top of their heads.

Onsen vary from quiet to noisy; some play piped music and often feature gushing fountains. Bathers will engage in conversation in this relaxed situation. There are usually prohibitions against rowdiness in the washing and bathing areas. A small amount of excess energy and splashing around is usually tolerated from children, however.

By 2015, around half (56%) of onsen operators had banned bathers with tattoos from using their facilities.[3][4][5] The original reason for the tattoo ban was to keep out Yakuza and members of other crime gangs who traditionally have elaborate full-body decoration.[6]

However, tattoo-friendly onsen do exist.[7] A 2015 study by the Japan National Tourism Organisation found that more than 30% of onsen operators at hotels and inns across the country will not turn someone with a tattoo away; another 13% said they would grant access to a tattooed guest under certain conditions, such as having the tattoo covered up.[3]

With the increase in foreign customers due to growing tourism, some onsens that previously banned tattoos are loosening their rules to allow guests with small tattoos to enter, provided they cover their tattoos with a patch or sticking plaster.[3][8]

Although millions of Japanese bathe in onsens every year with few noticeable side effects, there are still potential side effects to onsen usage, such as high blood pressure or heart disease.[10]

Legionella bacteria have been found in some onsens with poor sanitation.[11][12] Revelations of poor sanitary practices at some onsens have led to improved regulation by hot-spring communities to maintain their reputation.[13]

There have been reports of infectious disease found in hot bodies of water worldwide, such as various Naegleria species.[14] While studies have found the presence of Naegleria in hot spring waters, the worrisome Naegleria fowleri amoeba has not been identified.[14] Nevertheless, less than five cases have been seen historically in Japan, although not conclusively linked to onsen exposure.[15]

Many onsens display notices reminding anyone with open cuts, sores, or lesions not to bathe. Additionally, in recent years onsens are increasingly adding chlorine to their waters to prevent infection, although many onsen purists seek natural, unchlorinated onsens that do not recycle their water but instead clean the baths daily.[13] These precautions as well as proper onsen usage (i.e. not placing the head underwater, washing thoroughly before entering the bath) greatly reduce any overall risk to bathers.